Chapter 12.1

We try to predict which areas will have earthquakes by understanding

(1) frequencies and sizes of historic earthquakes, (2) geologic record of prehistoric earthquakes (3) tectonic settings of different regions.

what are some ways that volcanoes and magma cause Earthquakes?

1. An explosive volcanic eruption causes compression, transmitting energy as seismic waves 2. Volcanoes add tremendous weight to the crust, and this loading can lead to faulting and earthquakes 3. Many volcanoes have steep, unstable slopes underlain by rocks altered and weakened by hot water heated by magma. The flanks of such volcanoes can fall apart, causing landslides that shake the ground as they break away and travel down. Numerous small earthquakes also occur as the rocks break, prior to the actual landslide. 4. As magma moves beneath a volcano, it can push rocks out of the way, causing earthquakes. The emplacement of magma can cause a series of small and distinctive earthquakes, called volcanic tremors.

What Destruction can happen following an Earthquake?

1. Fire-Natural gas lines may rupture. Water lines also break, limiting the water to extinguish fires 2. Flooding- due to failure of dams as a result of ground rupturing 3. Uplift and subsidence of the land- causing areas that had been dry land before the earthquake to become inundated by seawater, flooding buildings and trees.

Rocks respond to stress in two ways

1. Flex and Bend 2. Break and Slip Both cause Earthquakes

Approach to forecasting earthquakes

1. measure patterns of seismic activity along a fault

How do Earthquake ruptures grow?

1. rupture starts on a small section of the fault below Earth's surface and begins to expand along the preexisting fault plane. Some rocks break adjacent to the fault, but most slip occurs on the actual fault surface, which is weaker than intact rock. 2. As the edge of the rupture migrates outward, it may eventually reach Earth's surface, causing a break called a fault scarp. the rupture migrates in both directions, but it may expand farther in one direction than in the other. 3. The rupture continues to grow along the fault plane and the fault scarp lengthens. The faulting relieves some of the stress, and rupturing will stop when the remaining stress can no longer overcome friction along the fault surface. At that point, the earthquake stops.

San Francisco, 1906

A huge earthquake occurred when the San Andreas fault ruptured. The earthquake ruptured the surface, leaving behind a series of cracks and open fissures. Within San Francisco, ground shaking destroyed most of the buildings. More than 3,000 people were killed and much of the city was devastated by fires that broke out.

How do earthquakes cause a tsunami?

An earthquake that occurs undersea or along coastal areas can generate a tsunami

Most deaths from earthquakes are due to?

Collapse of poorly constructed buildings

What to do during an earthquake

Drop! Cover! Hold on!

It increases gradually (sloping line), and then decreases abruptly (vertical line) when an earthquake occurs. This process is called the

Earthquake Cycle

What are some ways we can limit earthquake risks?

Earthquake Hazard Maps Computerized warning systems Stable buildings

seismicity

Earthquake activity

An Earthquake occurs when

Energy stored in rocks is suddenly released

What are some other causes of seismic waves?

Landslides and Explosions

Seismic Waves

Mechanical Energy that is transmitted through rocks as vibrations

What do Earthquakes do to a mountain?

Mountainous regions that undergo ground shaking may experience landslides, rock falls, and other earth movements

Most earthquakes are generated by

Movement along faults

hypocenter or focus

Place where earthquake is generated

world's best-known and most extensively studied fault

San Andrea's fault

What is released when an earthquake occurs?

Seismic waves

Earthquakes are produced when

Stress builds up along a fault over a long time, eventually causing the fault to slip.

What happened during the Great Alaskan Earthquake 1964?

THE SOUTHERN COAST OF ALASKA experienced one of the world's largest earthquakes in 1964. The earthquake, the strongest to have ever struck North America, destroyed buildings, triggered massive landslides, and unleashed a tsunami that caused damage and deaths from Alaska to California. The region also has active volcanoes, further evidence that this is a tectonically active area.

recurrence interval.

The average time between repeating earthquakes

Normal Fault

The crust is stretched horizontally, so earthquakes related to normal faults are most common along divergent plate boundaries, such as oceanic spreading centers, and in continental rifts.

Northridge, Los Angeles Area, 1994

This earthquake was generated by a thrust fault. The earthquake killed 60 people and caused $20 billion in damage. A section of freeway buckled, crushing the steel-reinforced concrete slabs. The thrust is not exposed on the surface, but when it ruptured it lifted up a large section of land.

where does the San Andrea's fault run along?

across California from the Mexican border to north of San Francisco

Aftershocks

are smaller earthquakes that occur after the main earthquake, but in the same area. Aftershocks occur because the main earthquake changes the stress around the hypocenter, and the crust adjusts to this change with more faulting. Aftershocks are very dangerous because they can collapse structures already damaged by the main shock. Aftershocks after a tsunami can cause widespread panic.

Most earthquakes occur by slip on a preexisting fault, but

but the entire fault does not begin to slip at once. Instead, the earthquake rupture starts in a small area (the hypocenter) and expands over time.

Before faulting, rocks

change shape (they strain) slightly as they are squeezed, pulled, and sheared. Once stress builds up to a certain level, slippage along a fault happens in a sudden, discrete jump. Faulting reduces the stress on the rocks, allowing some of the strained rocks to return to their original shapes.

Mexico City, 1985

earthquake occurred at a subduction zone. It damaged or destroyed many buildings in Mexico City and killed at least 9,500 people. Destruction was so extensive partly because Mexico City is built on lake sediments deposited in a bowlshaped basin.

Some earthquakes are caused by other human activities, such as the

filling of reservoirs behind dams and the disposal of wastewaters associated with the exploration for oil and gas.

Hebgen Lake, Yellowstone Area, 1959

generated by slip along a normal fault northwest of Yellowstone National Park. Ground shaking set loose the massive Madison Canyon slide, which buried 28 campers and formed a new lake, aptly named Earthquake Lake.

For the United States, the risk of earthquakes is greatest

in the most tectonically active areas, Western

Alaska, 1964

killed 128 people, triggered landslides, and collapsed parts of downtown Anchorage and nearby neighborhoods. This event was caused by thrust faults associated with the Aleutian Islands subduction zone. Most deaths and much damage were from a tsunami generated when a huge area of the seafloor was uplifted.

Ground shaking of unconsolidated, water-saturated sediment causes grains to lose grain-to grain contact. When this happens, the material loses most of its strength and begins to flow, a process called

liquefaction

Short-term prediction involves

monitoring the activity along an earthquake-prone fault

Most earthquakes occur along

narrow belts that coincide with Plate boundaries

epicenter

point on Earth's surface directly above where the earthquake occurs

Why do some earthquakes occur in the middle of continents?

ridge push

Seismic Stations

scientific instrument that scientists use to record the intensity and duration of waves

New Madrid, 1811-1812

series of large earthquakes generated over an ancient fault zone in the crust. The 1811-1812 earthquake death toll was relatively low because of the sparse population. The New Madrid zone has a high earthquake risk.

Charleston, 1886

the largest ever recorded in the southeastern United States. Buildings incurred some damage and 60 people died. The East Coast occasionally experiences earthquakes strong enough to be felt.

What can cause the "big earthquake" at the San Andreas Fault in the future?

the locked segment because it has not ruptured since 1857.

The risk of earthquake catastrophe depends on the

the number of people living in the region, how well the buildings are constructed, and individual and civic preparedness.

When the amount of stress equals

the strength of the fault, the fault slips, and the stress immediately decreases to the original level.

Faults slip because

the stress applied to them exceeds the ability of the rock to withstand the stress.

In strike-slip faults,

the two sides of the fault slip horizontally past each other. This can generate large earthquakes. Most strike-slip faults are near vertical, but some have moderate dips. The largest strike-slip faults are transform plate boundaries, like the San Andreas fault in California.

If the seismic event happens on the surface, such as during a human-caused surface explosion

then the epicenter and hypocenter are the same

Seismic waves travel

through the interior or along the surface of earth

Many large earthquakes are generated along reverse faults, especially the gently dipping variety called

thrust faults.

The emplacement of magma can cause a series of small and distinctive earthquakes, called

volcanic tremors.

Most active faults are in the

western states and most large earthquakes are in these same areas

Long-term forecasting is based mainly on the knowledge of

when and where earthquakes occurred in the past.

elastic behavior

when rocks return to their original shape after being strained

Direct damage from an earthquake results

when the ground shakes because of seismic waves, especially surface waves near the epicenter of the earthquake.

The probability that you will be affected by an earthquake depends on

where you live and whether that area experiences tectonic activity